Why does changing y in my conditional density function f(x|y) not change my results?

My assignment is to make plots of fx(x|y) for the cases y = {-5 0 5 10 15 20}. The conditional pdf is the result of a signal x going through gaussian noise n and coming out to the receiver y = x + n. When I run my code which I provided below with different values for y or yi in my program the plot never changes. Can anyone see what I'm doing wrong?

yi = -5;

F = zeros (1, 102);

e = repmat(2.71,1,51);

X1 = 0:0.1:5;

X2 = 5:0.1:10;

X = 0:0.1:10.1;

Fa = (X1/25) * (1/sqrt(8*3.14));

Fb = (2/5 - (X1/25)) * (1/sqrt(8*3.14));

P1 = (e.^((-1 * (yi-X1))/8));

P2 = (e.^((-1 * (yi-X2))/8));

F1 = Fa .* P1;

F2 = Fb .* P2;

k1 = trapz(X1,F1);

k2 = trapz(X2,F2);

F1 = F1 * (1/k1);

F2 = F2 * (1/k2);

j = 1;

while(j<=102)

    if j <= 51

        F(1,j) = F1(1,j);

    else          %f(x|y)

        F(1,j) = F2(1,j-51);

    end

    j = j + 1;

end

figure(1)

plot(X,F); %plot of f(x|y) vs x